Merge "Changing BluetoothHeadset APIs from @hide to @SystemApi" into stage-aosp-master am: 913ea0a167 am: 41dc0fb942 am: 712c9b7c3a am: 3ced5cdbb8

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/15728282

Change-Id: I6b093d3123bc9cced4cb7cddea9f1d327e097b29